The average life of the battery in the Time Timer Watch PLUS is approximately eight months, based on typical use. If the watch is used regularly (or left running overnight) on the repeat function while also using the vibrating alert, then the life of the battery will be significantly shorter due to the higher battery requirements for frequent vibrating alerts. If the watch starts to automatically reset itself to the original factory settings, then please replace the battery to stop the reset from occurring. The battery should also be replaced if there is no backlight or a dim display.